FRIDAY, March 21, 2014 (HealthDay News) -- Patient requests for specific drugs have a major influence on the medicines prescribed by doctors, a new study suggests.

The findings raise questions and concerns about how direct-to-consumer advertising for brand-name drugs affects medicine safety and costs, the researchers said.

The study authors created videos of actors who portrayed patients with two common, painful conditions. One was sciatica that caused back and leg pain, and the other was arthritis that caused knee pain.

In the videos, half of the "patients" with sciatica specifically requested a strong narcotic painkiller called oxycodone, and half of those with knee pain asked for the prescription drug Celebrex. The others requested "just something to make it better."

The videos were shown to 192 primary care doctors, who were then asked how they would treat the patients. About 20 percent of the sciatica patients who asked for oxycodone would have received it, compared with 1 percent of those who made no specific request, the investigators found.

The researchers noted that strong narcotic painkillers such as oxycodone are not generally recommended for sciatica.

About half of the knee arthritis patients who asked for Celebrex would have received it, compared with one-quarter of those who made no specific request. Celebrex is much more expensive than other drug options and provides no additional benefit, said study leader John McKinlay, of New England Research Institutes, and colleagues.

The authors said their findings, published in the April issue of the journal Medical Care, highlight the potential negative effects of direct-to-consumer advertising of drugs via so-called "Ask Your Doctor" ads.

The United States is one of only two countries that allow this type of controversial marketing, according to a journal news release.

"Supporters defend the practice as a way to empower consumers, while opponents argue that commercially motivated messages leads to inappropriate patient requests for medication," Dr. G. Caleb Alexander, deputy editor of Medical Care, said in the news release.

"In order to resolve this debate, more research is needed to determine the effects of [direct-to-consumer] advertising on patient and physician behavior, especially how it affects prescribing decisions and health outcomes," he said.